Answer #1: He is not that good. I get it. The guy is a journeyman who has bounced around from team to team (Houston, New England, Denver, Washington) for a reason.
But look at it another way: the guy has been in the NFL for 9 years, and in that time logged 440+ catches for 5600+ yards and 24 TDs for a reason too. The best thing about him: the guy is durable. He has only missed 5 games in his entire NFL career, all in 2006.
Another oddity: he seems to have had his best years since 2009, when he had the most catches, yards, and ypc (save his highest career ypc in 2004) of his career.
I don't see his getting cut loose by the Redskins as that big a deal. The Redskins do not have a true #1 like Steve Smith (their TE, Fred Davis, may lead the team in catches and yards) but some of their other WRs could get playing time in Carolina. So, who would you rather see as the team's #4 WR? Gaffney or Seyi Ajirotutu? I would pick the guy who can make plays between the hashmarks and had 50 first downs last year (and 42 the year before). The Panthers have Steve Smith and Louis Murphy to stretch the field and make big plays. That means that they are going to have to rely on Brandon LaFell, Kealoha Pilares and the other guys to move the sticks.
No, defenses aren't going to respect Gaffney, but he gives them another veteran option at #2 and #3 WR (he can play both possession and slot). The only downside is that it means pushing one of the younger guys off the roster, but if it makes the difference between getting to 10-6 and into the playoffs, that would be well worth the downside. Yes, I know David Gettis is still in the picture, but the Panthers can't wait 6 weeks.